Comparison of cytoplasmic and mitochondrial 4 s RNA from cultured hamster cells: Physical and metabolic properties

Abstract

Gel-electrophoretic, centrifugal and metabolic properties of the low molecular weight (“4 s”) mitochondrial RNA from cultured hamster cells have been compared with those of the corresponding cytoplasmic RNA fraction. On high-resolution acrylamide gel analysis the total 4 s RNA associated with the mitochondrial fraction ran as a single major peak. However, when “endogenous” (ethidium bromide-sensitive) mitochondrial patterns were constructed by subtracting the small amounts of contaminating cytoplasmic RNA present, a second discrete peak was observed, constituting approximately 3% of the ethidium-sensitive 4 s RNA, and migrating slightly beyond the main peak. No mitochondrial fraction with mobility resembling cytoplasmic 5 s RNA was detected. Mixed-labeling studies showed that the main mitochondrial 4 s component could be distinguished from cytoplasmic transfer RNA both by average mobility in gels and by sedimentation rate through sucrose gradients. The direction of the electrophoretic difference was a function of amperage, presumably reflecting differential effects of temperature on conformation. However, mitochondrial 4 s RNA consistently sedimented slower than cytoplasmic RNA, at both 5°C and 23°C, and under native or denaturing conditions. The centrifugal studies yielded an estimated average molecular weight for mitochondrial transfer RNA of 19,000. Studies involving short pulses and pulse-chases indicated that the mode of maturation of mitochondrial transfer RNA was qualitatively different from that of cytoplasmic transfer RNA. Under conditions in which as much as 75% of the 4 s label of the cytoplasmic fraction ran in gels as transfer RNA precursors, the corresponding pulse-labeled mitochondrial RNA migrated with mature transfer RNA.